The present invention relates to the improvement of belt arrangment reinforcing the crown portion of radial tires which are usefully applied to construction vehicles for running on rough ground.
Heretofore, these pneumatic radial tires, when running over rough ground having projections, are apt to have cuts in the tread portion, separation in side edge of the belt, cut of the belt cords and other problems. When these breakdowns are large, such tires cannot immediately be used and even when these breakdowns are small, the cut grows by subsequent use and such tires cannot be used to the end of tread life. Prior techniques proposed for improving these defects include (1) uniformly varying the angle of the entire belts, (2) varying the direction of cords in each belt layer, (3) varying the width, embedded cord number and number of the belts and (4) varying the thickness from the groove bottom of tread to the belt and the like. However, any of these processes cannot obtain satisfactory effects. Because radial tires heretofore developed for improved roads have not been designed to consider partial and noticeable forced deformation due to the ground projects, which become the problem when radial tires are applied to running on the rough ground, the above described proposals are not useful. Consequently belt breakage in the tire, the separation of cord ends in the belt from the circumferential rubber and the cut damage at the tread portion owing to rock stones and the like cannot be prevented and these tires are not fit to use on rough roads. Among these problems, the separation resistance at the cord end of belt and the cut resistance at the tread portion need solutions that are the converse of each other and it has been difficult to accomplish both performance where running on rough ground is possible without causing tread cut and further the running at a high speed is possible.
The present invention concurrently improves the cut resistance of the tread, the penetration resistance of the tread, the break resistance of the belt cord and the separation resistance at the belt cord end of radial tires to be used for running on rough ground, and the combination of the properties which have been heretofore considered to be imcompatible.
It has been considered that the separation resistance at the tire crown portion is influenced by the strain and heat generation when the tire is repeatedly subjected to load deformation and the penetrating force of rock stones. In this case, the higher the speed, the larger the load, the larger the road surface roughness and the higher the thickness of the tread portion, the more severe are the conditions required for the tire.
On the other hand, the cut resistance and the penetration resistance in the tire crown portion and the break resistance of the belt cords are greatly influenced by the flexibility of the crown portion, the thickness of the tread portion and the strength of the belt. In this case, when the crown portion is made harder, the thickness of the tread portion is reduced and the strength of the belt is lower. Consequently, more severe conditions are required for the tires.
The inventors have studied various belt structures and have found belt structures of radial tires for running on rough ground which are flexibly deformed even when running on rough ground and hardly cut. Such belt structures are low in heat generation even being used at high speed and separation at the cord end is not significant.